| The emergence of ammonia has made a major contribution to human development,and it has a large number of applications in important fields such as agricultural nitrogen fertilizer,chemical products and pharmaceutical preparation.N2 belongs to non-polar molecules,and the nitrogen-nitrogen triple bond(N?N)dissociation energy is very high.Therefore,how to use N2 ammonia efficiently has great significance.At present,the industrial nitrogen fixation is still based on the Haber-Bosch method,which the Fe/Ru-based material is used as a catalyst to synthesize ammonia under high temperature and high pressure using nitrogen and hydrogen as raw materials.Although this method of synthesizing ammonia has solved many living problems since its discovery,it also has great shortcomings.The main problem with industrial nitrogen fixation is excessive energy consumption and the discharge of large amounts of greenhouse gases.In order to solve this problem,some green nitrogen fixation methods have appeared in recent years,including photocatalytic nitrogen fixation and electrocatalytic nitrogen fixation.Photocatalytic nitrogen fixation is based on semiconductor materials under solar energy for synthesizing nitrogen and water to ammonia,which consumes less energy and is clean and pollution-free.However,the photocatalytic nitrogen fixation efficiency is far lower than the industrial nitrogen fixation efficiency.Studying photocatalytic nitrogen-fixing materials and catalytic mechanism to improve their nitrogen fixation efficiency is the focus of photocatalytic nitrogen fixation.Therefore,this thesis aims to improve the nitrogen-energy capacity of the material and the reduction electron transfer efficiency in the material catalysis process by modifying the titanium dioxide photocatalyst from the basic steps of the nitrogen reduction process,in order to improve the nitrogen fixation efficiency of the titanium dioxide,and further improve the photocatalytic mechanism,in-depth exploration will eventually provide new ideas for efficient nitrogen-fixing photocatalysts.The specific research contents are as follows:1.In order to improve the photocatalytic nitrogen fixation performance of TiO2 in pure water,the experiment was carried out by doping transition metal cobalt to improve the activation ability and electron transfer efficiency of TiO2.This experiment confirmed that the rutile phase TiO2 has higher nitrogen fixation performance than the anatase phase TiO2.On this basis,a two-step method was used to synthesize Co-load TiO2 composite.The oxygen vacancies(Ovac)on the surface of the catalyst have great nitrogen adsorption and activation potential,but how to change the overall performance of the catalyst under the introduction of oxygen vacancies is a big challenge.In addition,the doping of Co not only has a positive effect on the adsorption and activation of nitrogen,but also enhances the absorption of visible light and the efficiency of electron transport.The results of photocatalytic nitrogen fixation experiments show that the photocatalytic synthesis of ammonia by Co-load TiO2 reaches 35.19 ?mol g-1 h-1 after 2h of illumination,compared with pure TiO2,the ammonia yield is 12.64 ?mol g-1 h-1,which is nearly 3 times higher.It shows that the doped transition metal cobalt not only improves the utilization efficiency of photoelectrons,but also promotes the decomposition of water to provide a large number of protons for nitrogen fixation.In this paper,the performance of the catalysts was tested in pure water.Compared with the literature,the yield of synthetic ammonia in pure water is higher than that of many materials(FeAl@3DGraphene,Au/(BiO)2CO3,FeMoS-Chalcogels).The experimental results show that the efficiency of ammonia synthesis of TiO2 after Co loading has been greatly improved,and it has been able to maintain good photocatalytic performance after repeated cycles2.The Al layer is etched away by a strong acid etching precursor MAX(Ti3AlC2)to prepare a two-dimensional material MXenc(Ti3C2Tx).The MXene material has good electron transport efficiency,which facilitates electron transfer during nitrogen reduction,and the energy band and oxygen vacancies of titanium are favorable for adsorption and dissociation of N2.Therefore,the two-dimensional material MXene is sintered at different temperatures(200?,400 ?,600 ?,800 ?)under Ar atmosphere in a tube furnace.TiO2 is grown in situ on MXene to form MXene@TiO2 photocatalyst.TiO2 has strong light absorption and converts light energy into electrons.Then the electrons transfer to MXene and the nitrogen was reduced to ammonia.The nitrogen fixation experiment showed that the yield of ammonia under the air and nitrogen atmosphere for 2h was obviously improved.The nitrogen fixation yield in the air atmosphere was 25.39 pmol g-1 h-1 and the nitrogen retention performance was 88%after five cycles.The ammonia yield in the 0.05 M sulfuric acid(H2SO4)solution and visible light were 45.17 ?mol g-1 h-1 and 7.61 ?mol g-1 h-1,respectively.The experimental results show that the TiO2 particles are formed on the surface of the two-dimensional material by sintering MXene.Compared with the unsintered MXene(3.59 ?mol g-1 h-1),the yield of synthetic ammonia is increased by nearly 6.5 times. |
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